Positive electrode, secondary battery, battery pack, and vehicle

ABSTRACT

According to one embodiment, a positive electrode includes positive electrode active material particles, polymer fibers and inorganic solid particles. The polymer fibers have an average fiber diameter of 1 nm to 100 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe Japanese Patent Application No. 2017-178782, filed Sep. 19, 2017,the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a positive electrode, asecondary battery, a battery pack and a vehicle.

BACKGROUND

A nonaqueous electrolyte battery containing metallic lithium, a lithiumalloy, a lithium compound, or a carbonaceous material in a negativeelectrode is expected as a high-energy density battery. Thus, thisbattery has been extensively researched and developed. Lithium ionbatteries each including a positive electrode containing LiCoO₂,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, or LiMn₂O₄ as an active material and anegative electrode containing a carbonaceous material that allowslithium ions to be inserted and extracted have widely been put intopractice. A metal oxide or alloy has been studied as an alternative tothe carbonaceous material in the negative electrode.

Especially, when mounting the battery in a vehicle such as anautomobile, a material with excellent chemical and electrochemicalstability, strength, and corrosion resistance is needed as the materialof the negative electrode from the viewpoint of cycle performance undera high-temperature environment, high-output long-term reliability, andsafety. If high performance is required even in a cold district,high-output performance and long-life performance under alow-temperature environment are needed. On the other hand, from theviewpoint of improving safety performance, solid electrolytes,nonvolatile electrolytic solutions, and incombustible electrolyticsolutions have been developed as electrolytes. However, the solidelectrolytes, nonvolatile electrolytic solutions, and incombustibleelectrolytic solutions have not yet been put into practical use becausethey lower discharge rate performance, low-temperature performance, andlong-life performance.

In development of an all-solid-state secondary battery, research anddevelopment have been conducted to improve the ionic conductivity of anoxide solid electrolyte or sulfide solid electrolyte. However, the ionconduction resistance or reaction resistance in each electrode and theinterface resistance of the electrode/solid electrolyte are high,thereby imposing a problem concerning discharge performance andlow-temperature performance. In a charge-and-discharge cycle,disconnection of ion conduction paths in the electrodes caused byexpansion/contraction of the electrodes increases the resistance anddegrades the joining of the electrode/solid electrolyte. As a result,the interface resistance of the electrode/solid electrolyte increases,and the cycle life performance and discharge rate performancesignificantly lower. These problems make it difficult for theall-solid-state secondary battery using the solid electrolyte to be putinto practice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway sectional view showing a secondary batteryaccording to an embodiment;

FIG. 2 is a side view of the battery shown in FIG. 1;

FIG. 3 is a sectional view showing the secondary battery taken along thedirection perpendicular to a terminal extending direction according tothe embodiment;

FIG. 4 is an enlarged sectional view showing a portion A in FIG. 3;

FIG. 5 is a sectional view showing another example of the secondarybattery according to the embodiment;

FIG. 6 is a perspective view showing an example of a battery moduleincluding secondary batteries according to the embodiment;

FIG. 7 is an exploded perspective view showing a battery pack accordingto the embodiment;

FIG. 8 is a block diagram showing the electric circuit of the batterypack shown in FIG. 7;

FIG. 9 is a schematic view showing an example of a vehicle in which asecondary battery is mounted according to the embodiment; and

FIG. 10 is a view schematically showing another example of the vehicleaccording to the embodiment.

DETAILED DESCRIPTION

According to one embodiment, there is provided a positive electrodeincluding a positive electrode active material-containing layer. Thepositive electrode active material-containing layer includes positiveelectrode active material particles, polymer fibers, and inorganic solidparticles. The polymer fibers have an average fiber diameter of 1 nm to100 nm.

Another embodiment provides a secondary battery including the positiveelectrode of the embodiment.

Another embodiment provides a battery pack including the secondarybattery of the embodiment.

Another embodiment provides a vehicle including the battery pack of theembodiment.

First Embodiment

According to the first embodiment, there is provided a positiveelectrode containing positive electrode active material particles,polymer fibers, and inorganic solid particles. The average fiberdiameter of the polymer fibers ranges from 1 nm to 100 nm.

The positive electrode active material particles are those that allowlithium ions to be inserted and extracted. For example, high-potentialpositive electrode active material particles such as lithium nickelcobalt manganese composite oxide particles or lithium phosphate oxideparticles having an olivine structure significantly expand/contract atthe time of inserted/extracted lithium ions. Consequently, a liquidelectrolyte (electrolytic solution), a gel electrolyte, or the like heldby the positive electrode may ooze from the positive electrode due toexpansion/contraction of the positive electrode active material.

The polymer fibers having an average fiber diameter of 1 nm to 100 nmhave a nano-size fiber diameter, and are thus readily dispersed in thepositive electrode to form fine through holes arranged in a mesh shape.As a result, an electrolyte such as a liquid electrolyte or gelelectrolyte is carried or held by the polymer fibers. Thus, it ispossible to suppress disconnection of ion conduction caused byexpansion/contraction of the positive electrode active material, anddecrease the ion conduction resistance in the positive electrode.Therefore, a secondary battery including this positive electrode canimprove the charge-and-discharge cycle life, discharge rate performance,and low-temperature performance.

If the polymer fibers include cellulose fibers, the aspect ratio of thepolymer fibers can be set to, for example, a large value ranging from100 to 10,000, thereby encouraging formation of fine through holesarranged in a mesh shape by the polymer fibers. As a result, extractionof the electrolyte from the positive electrode is further suppressed,and it is thus possible to further decrease the ion conductionresistance in the positive electrode. Therefore, the secondary batteryincluding the positive electrode can further improve thecharge-and-discharge cycle life, discharge rate performance, andlow-temperature performance.

If the inorganic solid particles contain at least one material selectedfrom the group consisting of Al₂O₃, TiO₂, ZrO₂, and a phosphatecompound, they can conduct lithium ions without solvation. This makes itpossible to encourage ion conduction in the positive electrode.

The phosphate compound desirably contains a material having a NASICONstructure and expressed as Li_(1+y)Al_(x)M_(2−x)(PO₄)₃ (M represents atleast one element selected from the group consisting of Ti, Ge, Sr, Sn,Zr, and Ca for 0≤x≤1 and 0≤y≤1). It is considered that on an interfaceamong the positive electrode active material particles, the inorganicsolid particles, and the liquid or gel electrolyte held in the finethrough holes formed between the polymer fibers, lithium ionconcentration which is not influenced by solvation increases, andhigh-speed lithium ion conduction occurs, thereby decreasing the ionconduction resistance and the resistance of the positive electrodeactive material particle interface in the positive electrode.

When the content of the polymer fibers ranges from 0.2 wt % to 5 wt %,and the content of the inorganic solid particles ranges from 0.5 wt % to10 wt %, it is possible to decrease the lithium ion conductionresistance and the electron conduction resistance in the positiveelectrode while keeping the electrode density high.

Furthermore, a secondary battery including a positive electrode, anegative electrode containing a titanium-containing oxide, and anelectrolyte can obtain excellent charge-and-discharge cycle life,discharge rate performance, and low-temperature performance.

By including, in the titanium-containing oxide, at least one materialselected from the group consisting of a lithium titanium oxide having aspinel structure, a monoclinic titanium oxide, and a niobium titaniumoxide, it is possible to increase the safety of the secondary battery.

The positive electrode will be described in more detail below.

The positive electrode includes a positive electrode current collector,and a positive electrode active material-containing layer that iscarried by at least one principal surface, that is, one or both surfacesof the current collector. The positive electrode activematerial-containing layer contains positive electrode active materialparticles that allow lithium ions to be inserted/extracted, polymerfibers having an average fiber diameter of 1 nm to 100 nm, and inorganicsolid particles. The positive electrode active material-containing layermay also contain a conductive agent and a binder.

An aluminum foil or aluminum alloy foil can be used as the positiveelectrode current collector. The purity of the aluminum foil or aluminumalloy foil can be set to 99% or higher. The upper limit of the purity is100% which is pure aluminum. A more preferable aluminum purity rangesfrom 99% to 99.99%. In this range, it is possible to reducedeterioration in high-temperature cycle life caused by dissolution of animpurity element.

An aluminum alloy is preferably an alloy containing, in addition toaluminum, one or more elements selected from the group consisting ofiron, magnesium, zinc, manganese, and silicon. For example, an Al—Febased alloy, Al—Mn based alloy, and Al—Mg based alloy can obtain astrength higher than that of aluminum. On the other hand, the contentsof a transition metal such as nickel or chromium in an aluminum alloyand aluminum are preferably 100 wt ppm or less (including 0 wt ppm). Forexample, an Al—Cu alloy improves the strength but degrades the corrosionresistance.

Examples of the positive electrode active material include a lithiummanganese composite oxide, a lithium cobalt composite oxide, a lithiumnickel composite oxide, a lithium nickel cobalt composite oxide, alithium cobalt aluminum composite oxide, a lithium nickel aluminumcomposite oxide, a lithium manganese nickel composite oxide having aspinel structure, a lithium manganese cobalt composite oxide, alithium-containing phosphate compound having an olivine structure, afluorinated iron sulfate, and a lithium nickel cobalt manganesecomposite oxide. One type or two or more types of positive electrodeactive materials can be used.

As the lithium manganese composite oxide, for example, LiMn₂O₄ (0<x≤1),Li_(x)MnO₂ (0<x≤1), or the like can be used.

As the lithium cobalt composite oxide, for example, Li_(x)CoO₂ (0<x≤1)or the like can be used.

As the lithium nickel aluminum composite oxide, for example,Li_(x)Ni_(1−y)Al_(y)O₂ (0<x≤1, 0<y≤1) or the like can be used.

As the lithium nickel cobalt composite oxide, for example,Li_(x)Ni_(1−y−z)Co_(y)Mn_(z)O₂ (0<x≤1, 0<y≤1, 0 z≤1, 0<1−y−z<1) or thelike can be used.

As the lithium manganese cobalt composite oxide, for example,Li_(x)Mn_(y)Co_(1−y)O₂ (0<x≤1, 0<y<1) or the like can be used.

As the lithium manganese nickel composite oxide having the spinelstructure, for example, Li_(x)Mn_(2−y)Ni_(y)O₄ (0<x≤1, 0<y<2) or thelike can be used.

As the lithium-containing phosphate compound having the olivinestructure, for example, Li_(x)FePO₄ (0<x<1), Li_(x)Fe_(1−y)Mn_(y)PO₄(0<x≤1, 0≤y≤1), Li_(x)CoPO₄ (0<x≤1), Li_(x)MnPO₄ (0<x≤1), or the likecan be used.

As the fluorinated iron sulfate, for example, Li_(x)FeSO₄F (0<x≤1) orthe like can be used.

As the lithium nickel cobalt manganese composite oxide, for example,Li_(x)Ni_(1−y−z)CO_(y)Mn_(z)O₂ (0<x≤1.1, 0<y≤0.5, 0<z s 0.5, 0<1−y−z<1)or the like can be used.

With the above positive electrode active materials, it is possible toobtain a high positive electrode voltage. Among them, the lithium nickelaluminum composite oxide, lithium nickel cobalt manganese compositeoxide, and lithium manganese cobalt composite oxide can suppress areaction with an electrolyte under a high-temperature environment andgreatly improve the battery life. The lithium nickel cobalt manganesecomposite oxide expressed as Li_(x)Ni_(1−y−z)Co_(y)Mn_(z)O₂ can obtain along life under high temperature.

The positive electrode active material particles may be primaryparticles, secondary particles as the aggregates of primary particles,or a mixture of primary particles and secondary particles.

The average primary particle size of the positive electrode activematerial particles can range from 0.05 m to 3 μm.

The average secondary particle size of the positive electrode activematerial particles can range from 3 μm to 20 μm.

The conductive agent can improve the electron conductivity and suppressthe contact resistance to a current collector. As the conductive agent,for example, acetylene black, carbon black, or graphite can be used.

The binder can bind the active material and the conductive agent. As thebinder, for example, a polymer such as polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), or fluorine-based rubber can be used.

The reason why the average fiber diameter of the polymer fibers isspecified will be described. If the average fiber diameter exceeds 100nm, it is difficult to distribute the polymer fibers to fine portions inthe positive electrode. In addition, since the specific surface area ofthe polymer fibers is short, the amount of the electrolyte held by thepolymer fibers becomes small. Consequently, the low-temperatureperformance or discharge rate performance of the battery lowers. Theaverage fiber diameter is desirably small. However, if the average fiberdiameter is smaller than 1 nm, dispersion of the fibers in the electrodeis insufficient, and the fibers thus cannot uniformly hold theelectrolyte. The average fiber diameter more preferably ranges from 5 nmto 50 nm.

The content of the polymer fibers in the positive electrode activematerial-containing layer desirably ranges from 0.2 wt % to 5 wt %. Ifthe content of the polymer fibers exceeds 5 wt %, the contents of theremaining components relatively decrease, and thus the density of thepositive electrode may lower. If the content of the polymer fibers isset to be smaller than 0.2 wt %, the effect of the polymer fibers havingan average fiber diameter of 1 nm to 100 nm cannot be obtained, therebydegrading the charge-and-discharge cycle life, discharge rateperformance, and low-temperature performance. The content morepreferably ranges from 0.3 wt % to 2 wt %.

The polymer fibers are preferably cellulose fibers (cellulosenanofibers). The cellulose fibers have an average fiber diameter of 1 nmto 100 nm, that is, a nano-size fiber diameter, and a very high aspectratio (100 to 10,000), and can thus firmly hold a liquid electrolyte inthe fine through holes in the positive electrode.

The inorganic solid particles desirably contain at least one materialselected from the group consisting of Al₂O₃, TiO₂, ZrO₂, and a phosphatecompound. As the phosphate compound, for example, AlPO₄, Li₃PO₄, amaterial having a NASICON structure and expressed asLi_(1+y)Al_(x)M_(2−x)(PO₄)₃ (M represents at least one element selectedfrom the group consisting of Ti, Ge, Sr, Sn, Zr, and Ca for 0≤x≤1 and0≤y≤1) can be used. The phosphate compound having the NASICON structureand expressed as Li_(1+y)Al_(x)M_(2−x)(PO₄)₃ has excellent stability inthe atmosphere, and high lithium ion conductivity.

The average particle size of the inorganic solid particles can be set to1 μm or less. The average particle size more preferably ranges from 0.01μm to 1 μm, and much more preferably 0.05 μm to 0.6 μm. In this range,the lithium ion conductivity in the positive electrode is improved,thereby decreasing the resistance in the positive electrode. Bycompositing the polymer fibers and the inorganic solid particles,high-speed ionic conductivity can be maintained even in the positiveelectrode that significantly expands/contracts. It is considered thatespecially on an interface among the high-potential positive electrodeactive material particles, the inorganic solid particles having anaverage particle size of 1 μm or less, and the liquid or gel electrolyteheld in the fine through holes arranged between the polymer fibers, thelithium ion concentration which is not influenced by solvationincreases, and high-speed lithium ion conduction occurs, therebydecreasing the ion conduction resistance and the resistance of thepositive electrode active material interface in the positive electrode.

The content of the inorganic solid particles in the positive electrodeactive material-containing layer preferably ranges from 0.5 wt % to 10wt %. If the content of the inorganic solid particles exceeds 10 wt %,the contents of the remaining components relatively decrease, and thusthe density of the positive electrode may lower. If the content of theinorganic solid particles is set to be smaller than 0.5 wt %, the effectof the inorganic solid particles cannot be obtained, thereby degradingthe charge-and-discharge cycle life, discharge rate performance, andlow-temperature performance. The content more preferably ranges from 1wt % to 6 wt %.

As the polymer used to make a gel of the electrolyte in the positiveelectrode, for example, a polymer that produces a gel electrolyte with apresence of carbonate or the like can be used. Examples of the polymerinclude polyacrylonitrile (PAN), polyethylene oxide (PEO),polyvinylidene fluoride (PVdF), and polymethyl metacrylate.

The gel polymer electrolyte using polyacrylonitrile is preferable sinceit has high ionic conductivity to improve the discharge performance andlow-temperature performance. The ratio of the gel polymer electrolyte tothe positive electrode active material-containing layer preferablyranges from 1 wt % to 10 wt %. If the ratio falls outside the range, thelow-temperature performance or discharge performance may lower.

One type or two or more types of components such as the positiveelectrode active material, conductive agent, binder, polymer fibers,inorganic solid particles, and polymer can be contained in theabove-described positive electrode.

As the mixing ratio of the positive electrode active material, theconductive agent, and the binder, the content of the positive electrodeactive material preferably ranges from 80 wt % to 95 wt %, the contentof the conductive agent preferably ranges from 3 wt % to 18 wt %, andthe content of the binder preferably ranges from 2 wt % to 7 wt %. Theconductive agent can exhibit the above-described effect at 3 wt % ormore, and can reduce decomposition of the nonaqueous electrolyte on theconductive agent surface stored under a high temperature at 18 wt % orless. The binder can obtain a sufficient electrode strength at 2 wt % ormore, and can reduce the insulating portion of the electrode at 7 wt %or less.

The positive electrode is produced by, for example, suspending thepositive electrode active material, the conductive agent, and the binderin an appropriate solvent, applying the suspended substance to thepositive electrode current collector, drying the suspended substance,and performing press. The positive electrode press pressure preferablyranges from 0.15 ton/mm to 0.3 ton/mm. This range is preferable becausethe adhesion or peel strength between the positive electrode activematerial-containing layer and the positive electrode current collectorsuch as an aluminum foil increases, and the elongation percentage of apositive electrode current collector foil becomes 20% or less.

A method of measuring the content of the polymer fibers in the positiveelectrode active material-containing layer will be described below. Thepositive electrode active material-containing layer peeled from thepositive electrode current collector is ground using a mortar or thelike, and the thus obtained ground product is dispersed in water toseparate the polymer fibers using a specific gravity difference. Thepolymer fibers in a supernatant are dried at 100° C. for 12 hours, andthe weight of the polymer fibers is measured, thereby obtaining thecontent of the polymer fibers in the positive electrode activematerial-containing layer.

The polymer fibers whose weight has been measured by the above-describedmethod are observed by an SEM (Scanning Electron Microscope) at amagnification of 10,000, and widths are measured at positions of 25%,50%, and 75% of the full length of the polymer fibers within the fieldof view. The average of the values of the measured widths is obtained asan average fiber diameter. Measurement is performed for all the polymerfibers existing within the field of view.

A method of measuring the content of the inorganic solid particles inthe positive electrode active material-containing layer will bedescribed below. The positive electrode active material-containing layerhaving undergone an electrolyte removal treatment by a method (to bedescribed later) is ground using a mortar or the like, and the thusobtained ground product is dispersed in water to separate the polymerfibers and the conductive agent using a specific gravity difference. Thepolymer fibers and conductive agent in a supernatant are removed. Theprecipitate is washed by dimethyl carbonate to remove the binder.Subsequently, the obtained material is dried, and particle sizedistribution data is obtained for the thus obtained powder by laserdiffraction (to be described later). Based on the obtained particle sizedistribution data, the content of the inorganic solid particles in thepositive electrode active material-containing layer is calculated.

The average primary particle sizes of the positive electrode activematerial particles and the inorganic solid particles are measured by thefollowing method. That is, a laser diffraction particle-size analyzer(SALD-300 of Shimadzu corporation or an apparatus having the equivalentfunction) is used to perform measurement by a method of adding about 0.1g of a sample, a surfactant, and 1 to 2 mL of distilled water to abeaker, sufficiently stirring them, injecting the mixture into a waterstirring tank, measuring a luminous intensity distribution 64 times atan interval of 2 sec, and analyzing particle size distribution data.

With respect to the positive electrode included in the secondarybattery, if the average fiber diameter of the polymer fibers and thelike are confirmed, the positive electrode is extracted from thesecondary battery by the following method. That is, the secondarybattery is disassembled in a glove box filled with argon to extract thepositive electrode. The positive electrode active material-containinglayer is separated from the positive electrode current collector of theextracted positive electrode. The positive electrode activematerial-containing layer is washed by dimethyl carbonate to remove theelectrolyte, and dried. After that, the average fiber diameter of thepolymer fibers and the like are confirmed.

The positive electrode according to the above-described first embodimentincludes the positive electrode active material particles that allowlithium ions to be inserted/extracted, the polymer fibers having anaverage fiber diameter of 1 nm to 100 nm, and the inorganic solidparticles. This can decrease the ion conduction resistance in thepositive electrode. Therefore, the secondary battery including thispositive electrode can improve the charge-and-discharge cycle life,discharge rate performance, and low-temperature performance.

Second Embodiment

A secondary battery according to the second embodiment includes apositive electrode, a negative electrode, and an electrolyte. As thepositive electrode, the positive electrode according to the firstembodiment can be used. The secondary battery may include a containermember that stores the positive electrode, the negative electrode, andthe electrolyte. In the secondary battery, an electrolyte-containinglayer may be arranged between the positive and negative electrodes.

The electrolyte, the negative electrode, the electrolyte-containinglayer, and the container member will be described below.

1) Electrolyte

Examples of the electrolyte are a liquid electrolyte, a gel electrolyte,a solid electrolyte, and a composite electrolyte.

The liquid electrolyte contains an electrolyte salt such as a lithiumsalt and an organic solvent in which the electrolyte salt is dissolved.The gel electrolyte contains an electrolyte salt such as a lithium salt,an organic solvent in which the electrolyte salt is dissolved, and apolymer as a gel agent. The solid electrolyte contains an oxide orsulfide having lithium ion conductivity. The composite electrolytecontains inorganic solid particles (to be referred to as secondinorganic solid particles hereinafter), and a polymer containing lithiumions and an organic solvent, and is a composite of the second inorganicsolid particles and gel electrolyte. The composite electrolyte ispreferable because it has high ionic conductivity.

The polymer is not particularly limited as long as it can produce a gelcontaining the lithium ion-containing organic solvent, and either achemical gel agent or a physical gel agent can be used. For example, apolymer that can make a gel electrolyte with a presence of carbonatesuch as polyacrylonitrile (PAN), polyethylene oxide (PEO),polyvinylidene fluoride (PVdF), or polymethyl metacrylate can be used.When a polymer electrolyte is generated by compositing the carbonate andthe polymer and followed by producing a gel, the ionic conductivity ofthe composite electrolyte is improved. The gel polymer electrolytecontaining polyacrylonitrile has high ionic conductivity, and ispreferable to improve the discharge performance and low-temperatureperformance. The ratio of the polymer to the composite electrolytepreferably ranges from 1 wt % to 10 wt %. If the ratio falls outside therange, the low-temperature performance or discharge performance maylower. One type or two or more types of polymers can be used.

The composite electrolyte may contain a binder to improve the mechanicalstrength. Examples of the binder are polyvinylidene fluoride (PVdF) andan acrylic binder. The binder may be added within the range of 0 to 5 wt%. If this range is exceeded, the ionic conductivity of the electrolytemay lower, thereby degrading the discharge performance.

The composite electrolyte may contain the same kind of polymer fibers asthe polymer fibers contained in the positive electrode according to thefirst embodiment. The content of the polymer fibers in the compositeelectrolyte preferably ranges from 0.1 wt % to 10 wt %. In this range,the mechanical strength of the composite electrolyte layer increases,thereby making it possible to decrease the thickness to 10 μm. Thus, itis possible to largely reduce an internal short circuit andself-discharge while decreasing the internal resistance of the battery.

Furthermore, the joining strength between the composite electrode andthe positive and negative electrodes become large, and do not degrade ina charge-and-discharge cycle, and a rise in interface resistance issuppressed, thereby largely improving the cycle life performance.

The secondary inorganic solid particles may be inorganic particleshaving no lithium ion conductivity or low lithium ion conductivity, oran inorganic solid electrolyte having high lithium ion conductivity. Onetype or two or more types of second inorganic solid particles can beused.

As the inorganic particles having no lithium ion conductivity or lowlithium ion conductivity, lithium aluminum oxide (for example, LiAlO₂ orLi_(x)Al₂O₃ for 0<x≤1), lithium silicon oxide, or lithium zirconiumoxide can be used.

An example of the inorganic solid electrolyte having lithium ionconductivity includes an oxide solid electrolyte having a garnetstructure. The oxide solid electrolyte having the garnet structure hashigh lithium ion conductivity and reduction resistance, therebyproviding the advantage of a wide electrochemical window. Examples ofthe oxide solid electrolyte having the garnet structure includeLi_(5+x)A_(x)La_(3−x)M₂O₁₂ (A represents at least one element selectedfrom the group consisting of Ca, Sr, and Ba, M represents Nb and/or Ta,and x is preferably equal to 0.5 or less (including 0)), Li₃M_(2−x)L₂O₁₂(M represents Nb and/or Ta, L includes Zr, and x is preferably equal to0.5 or less (including 0)), Li_(7-3x)Al_(x)La₃Zr₃O₁₂ (x is preferablyequal to 0.5 or less (including 0)), and Li₇La₃Zr₂O₁₂. Among them,Li_(6.25)Al_(0.25)La₃Zr₃O₁₂, Li_(6.4)La₃Zr_(1.4)Ta_(0.6)O₁₂,Li_(6.4)La₃Zr_(1.6)Ta_(0.6)O₁₂, and Li₇La₃Zr₂O₁₂ have high ionicconductivity, and are electrochemically stable, and thus have excellentdischarge performance and cycle life performance. Furthermore, fineparticles having a specific surface area of 10 m²/g to 500 m²/g(preferably, 50 m²/g to 500 m²/g) have the advantage of being chemicallystable with respect to an organic solvent.

An example of the inorganic solid electrolyte having lithium ionconductivity includes a lithium phosphate solid electrolyte having aNASICON structure. The lithium phosphate solid electrolyte having theNASICON structure has high stability to water, and is thus difficult tobe eluted. An example of the lithium phosphate solid electrolyte havingthe NASICON structure includes Li_(1+y)Al_(x)M_(2−x)(PO₄)₃ where Mrepresents at least one element selected from the group consisting ofTi, Ge, Sr, Sn, Zr, and Ca, and x and y satisfy 0≤x≤1 and 0≤y≤1,respectively. Preferable examples are Li_(1+x)Al_(x)Ge_(2−x)(PO₄)₃,Li_(1+x)Al_(x)Zr_(2−x)(PO₄)₃, and Li_(1+x)Al_(x)Ti_(2−x) (PO₄)₃ where xpreferably ranges from 0 to 1. Each of the exemplified solidelectrolytes has high ionic conductivity and high electrochemicalstability. Each solid electrolyte is electrochemically stable, and thushas excellent discharge performance and cycle life performance. Even ifthe solid electrolyte is further atomized to have a specific surfacearea of 10 m²/g to 500 m²/g, it has the advantage of being chemicallystable with respect to an organic solvent. A preferable particle size(diameter) ranges from 0.01 μm to 1 μm. In this range, the ionicconductivity of the composite electrolyte is increased, therebyimproving the discharge performance and low-temperature performance. Theparticle size more preferably ranges from 0.05 μm to 0.6 μm.

The ratio of the inorganic solid particles to the composite electrolytecan range from 85 wt % to 98 wt %. By setting the ratio to 85 wt % ormore, it is possible to avoid an increase in self-discharge along withan internal short circuit caused by a decrease in strength of theelectrolyte. By setting the ratio to 98 wt % or less, it is possible toavoid degradation in discharge performance or low temperatureperformance caused by a sudden reduction in ionic conductivity of theelectrolyte.

Both the lithium phosphate solid electrolyte having the NASICONstructure and the oxide solid electrolyte having the garnet structuremay be used as inorganic solid electrolytes having lithium ionconductivity.

The lithium ion-containing organic electrolytic solution is prepared bydissolving, for example, a lithium salt in a solvent including anorganic solvent.

Examples of the lithium salt are LiPF₆, LiClO₄, LiBF₄, LiCF₃SO₃,LiN(FSO₂)₂, and LiN(CF₃SO₂)₂. At least one type of lithium salt selectedfrom the group consisting of LiPF₆, LiBF₄, and LiN(FSO₂)₂ is improved indischarge performance by increasing the ionic conductivity.

The organic solvent desirably includes carbonates. Examples of thecarbonates are a cyclic carbonate such as ethylene carbonate (EC),propylene carbonate (PC) and butylene carbonate (BC), a chain carbonatesuch as diethyl carbonate (DEC), methyl ethyl carbonate (MEC) anddimethyl carbonate (DMC). The use of propylene carbonate (PC), diethylcarbonate (DEC), or methyl ethyl carbonate (MEC) improves thelow-temperature performance.

The organic solvent can include solvents other than carbonates. Examplesare γ-butyrolactone (GBL), α-methyl-γ-butyrolactone (MBL), phosphoricacid esters (for example, trimethyl phosphate (PO(OCH₃)₃), triethylphosphate (PO(OC₂H₅)₃), tripropyl phosphate (PO(OC₃H₇)₃), and tributylphosphate (PO(OC₄H₉)₃)). Especially, the use of γ-butyrolactone ortrimethyl phosphate suppresses a rise in ion conduction resistance undera low-temperature environment, thereby making it possible to improve thedischarge performance under the low-temperature environment (−30° C. orlower).

The composite electrolyte may be a gel electrolyte. By compositing alithium ion-containing organic electrolytic solution with a polymer, agel electrolyte can be obtained. The gel electrolyte can cover at leastpart of the inorganic solid particle surface.

The gel composite electrolyte is obtained by, for example, mixing, withthe second inorganic solid particles and/or polymer fibers, anelectrolyte composition containing the lithium ion-containing organicelectrolytic solution and the polymer, as needed, and performing a heattreatment, as needed.

It is confirmed by the following method that the composite electrolyteis a gel. A gel can be confirmed by applying a pressure of 10 g/cm² tothe composite electrolyte, and checking whether the electrolyticsolution oozes or not.

One type or two or more types of components such as the lithium salt,organic solvent, polymer, polymer fibers, inorganic solid particles, andbinder can be contained in the above-described electrolyte.

2) Negative Electrode

The negative electrolyte includes a negative electrode current collectorand a negative electrode active material-containing layer carried by atleast one principal surface, that is, one or both surfaces of thecurrent collector. The negative electrode active material-containinglayer contains an active material, and can also contain a conductiveagent and a binder.

An aluminum foil or aluminum alloy foil can be used as the negativeelectrode current collector. The purity of the aluminum foil or aluminumalloy foil can be set to 98% or higher. The upper limit of the purity is100% which is pure aluminum. A more preferable aluminum purity rangesfrom 98% to 99.95%. When a negative electrode active material containingtitanium-containing oxide particles is used to reduce the negativeelectrode press pressure, it is possible to suppress elongation of thenegative electrode current collector having the above-described purity.As a result, it is possible to improve the electron conductivity in thenegative electrode current collector, and produce a low-resistancenegative electrode by suppressing crush of the secondary particles ofthe titanium-containing oxide.

An aluminum alloy is preferably an alloy containing, in addition toaluminum, one or more elements selected from the group consisting ofiron, magnesium, zinc, manganese, and silicon. For example, an Al—Febased alloy, Al—Mn based alloy, and Al—Mg based alloy can obtain astrength higher than that of aluminum. On the other hand, the contentsof a transition metal such as nickel or chromium in an aluminum alloyand aluminum are preferably 100 wt ppm or less (including 0 wt ppm). Forexample, an Al—Cu based alloy improves the strength but degrades thecorrosion resistance.

The negative electrode active material is not particularly limited aslong as it is possible to allow lithium ions to be inserted/extracted,and includes a carbon material, graphite material, lithium alloy, metaloxide, and metal sulfide. One type or two or more types of negativeelectrode active materials can be used. A negative electrode activematerial containing a titanium-containing oxide is preferable. When atitanium-containing oxide is used, an aluminum foil or aluminum alloyfoil can be used as a negative electrode current collector in place of acopper foil, thereby reducing the weight and the cost. Additionally,this is advantageous in a bipolar electrode structure. Theinsertion/extraction potential of the lithium ions of thetitanium-containing oxide preferably ranges from 1 V to 3 V (vs. Li/Li⁺)relative to the Li potential. Examples of the titanium-containing oxidesatisfying the above condition include a lithium titanium oxide, atitanium oxide, a niobium titanium oxide, and a sodium niobium titaniumoxide. The titanium-containing oxide desirably includes at least onematerial selected from the group consisting of a lithium titanium oxidehaving a spinel structure, a monoclinic titanium oxide, and a niobiumtitanium oxide.

Examples of the lithium titanium oxide include a lithium titanium oxidehaving a spinel structure (for example, the general formula isLi_(4+x)Ti₅O₁₂ (−1≤x≤3)), a lithium titanium oxide having a ramsdellitestructure (for example, Li_(2+x)Ti₃O₇ (−1≤x≤3)), Li_(1+x)Ti₂O₄ (0≤x≤1),Li_(1.1+x)Ti_(1.8)O₄ (0≤x≤1), and Li_(1.07+x)Ti_(1.86)O₄ (0≤x≤1).

Examples of the titanium oxide include a titanium oxide having amonoclinic crystal structure (for example, the structure before chargeis TiO₂(B), Li_(x)TiO₂ (0≤x)), a titanium oxide having a rutilestructure (for example, the structure before charge is TiO₂, Li_(x)TiO₂(0≤x)), and a titanium oxide having an anatase structure (for example,the structure before charge is TiO₂, Li_(x)TiO₂ (0≤x)).

Examples of the niobium titanium oxide include a material expressed asLi_(a)TiM_(b)Nb_(2+β)O_(7+σ) (0≤a≤5, 0≤b≤0.3, 0≤β≤0.3, 0≤σ≤0.3, M is atleast one element selected from the group consisting of Fe, V, Mo, andTa).

Examples of the sodium niobium titanium oxide include an orthorhombicNa-containing niobium titanium composite oxide represented by thegeneral formula Li_(2+v)Na_(2-w)M1_(x)Ti_(6−y−z)Nb_(y)M2_(z)O_(14+δ)(0≤v≤4, 0<w<2, 0≤x<2, 0<y≤6, 0≤z<3, −0.5≤δ≤0.5, 0<6−y−z<6, M1 includesat least one material selected from the group consisting of Cs, K, Sr,Ba, and Ca, and M2 includes at least one material selected from thegroup consisting of Zr, Sn, V, Ta, Mo, W, Fe, Co, Mn, and Al).

A preferable negative electrode active material is a lithium titaniumoxide having a spinel structure. The lithium titanium oxide having thespinel structure has a small volume change at the time ofcharge/discharge. In addition, an aluminum foil or aluminum alloy foilcan be used as a negative electrode current collector in place of acopper foil, thereby reducing the weight and the cost. This isadvantageous in a bipolar electrode structure. The ratio of a negativeelectrode active material other than the titanium-containing oxide tothe whole negative electrode active material is desirably 50 wt % orless.

The particles of the titanium-containing oxide can include singleprimary particles, secondary particles as the aggregates of primaryparticles, or a mixture of single primary particles and secondaryparticles.

The average particle size (diameter) of the secondary particles can beset to 2 μm or more, preferably, larger than 5 μm. The average particlesize more preferably ranges from 7 μm to 20 μm. In this range, anegative electrode having a high density can be produced while keeping anegative electrode press pressure low, and it is possible to suppresselongation of the aluminum-containing current collector. The secondaryparticles of the titanium-containing oxide are obtained, as follows.That is, for example, the raw materials of the active material arereactively synthesized to produce an active material precursor having anaverage particle size of 1 μm or less. After that, a firing treatment isperformed, and a grinding treatment is performed using a grinder such asa ball mill or a jet mill. Next, in a firing treatment, the activematerial precursor is aggregated to grow secondary particles with alarger particle size.

The average particle size (diameter) of the primary particles isdesirably 1 μm or less. The effect is remarkable in high inputperformance (rapid charge). This is because, for example, the diffusiondistance of lithium ions in the active material is shortened and thespecific surface area is increased. A more preferable average particlesize ranges from 0.1 μm to 0.8 μm. The negative electrode activematerial-containing layer may contain a mixture of the primary particlesand secondary particles of the titanium-containing oxide. In terms ofimproving the density, the negative electrode active material-containinglayer preferably contains 5 vol % to 50 vol % of primary particles.

At least part of the titanium-containing oxide particle surface isdesirably covered with a carbon material layer. This can reduce thenegative electrode resistance. A carbon material precursor is added in asecondary particle manufacturing process, and fired at 500° C. or higherin an inert atmosphere, thereby making it possible to cover at leastpart of the titanium-containing oxide particle surface with the carbonmaterial layer.

The titanium-containing oxide particles have an average primary particlesize of 1 μm or less, and the specific surface area in the BET method byN₂ adsorption preferably ranges from 3 m²/g to 200 m²/g. This canfurther improve the affinity of the negative electrode for anelectrolyte.

The specific surface area of the negative electrode preferably rangesfrom 3 m²/g to 50 m²/g, more preferably, 5 m²/g to 50 m²/g, and muchmore preferably, 1 m²/g to 20 m²/g. In this range, reductivedecomposition of an electrolyte under a high-temperature environment issuppressed, thereby improving the cycle life. The specific surface areof the negative electrode indicates the surface area of the negativeelectrode active material-containing layer per g (except the weight ofthe current collector). Note that the negative electrode activematerial-containing layer can be a porous layer containing a negativeelectrode active material, a conductive agent, and a binder.

The porosity of the negative electrode (except the current collector)desirably ranges from 20% to 50%. A negative electrode having a highaffinity for an electrolyte and also having a high density can thus beobtained. The porosity more preferably ranges from 25% to 40%.

As the conductive agent, for example, a carbon material, metal compoundpowder, or metal powder can be used. Examples of the carbon material areacetylene black, carbon black, coke, carbon fiber, and graphite. The BETspecific surface area of the carbon material by N₂ adsorption ispreferably 10 m²/g or more. Examples of the metal compound powderinclude TiO, TiC, and TiN powders. Examples of the metal powder includeAl, Ni, Cu, and Fe powders. Preferable examples of the conductive agentinclude coke having an average particle size of 10 μm or less andobtained by a heat-treatment at a temperature of 800° C. to 2,000° C.,graphite, acetylene black, carbon fiber having an average fiber diameterof 1 μm or less, and a TiO powder. One or more materials selected fromthem can reduce the electrode resistance and improve the cycle lifeperformance. One type or two or more types of conductive agents can beused.

Examples of the binder are polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), fluorine-based rubber, acrylic rubber,styrene butadiene rubber, a core shell binder, and polyimide. One typeor two or more types of binders can be used.

One type or two or more types of components such as the negativeelectrode active material, conductive agent, and binder can be containedin the above-described negative electrode.

As the mixing ratio of the negative electrode active material, theconductive agent, and the binder, the content of the negative electrodeactive material preferably ranges from 80 wt % to 95 wt %, the contentof the conductive agent preferably ranges from 3 wt % to 18 wt %, andthe content of the binder preferably ranges from 2 wt % to 7 wt %.

The negative electrode is produced by, for example, suspending particlesof the negative electrode active material, the conductive agent, and thebinder in an appropriate solvent, applying the suspended substance tothe current collector, drying the suspended substance, and performingpress (for example, heating press).

The average primary particle size of the negative electrode activematerial is measured by the following method. That is, a laserdiffraction particle-size analyzer (SALD-300 of Shimadzu corporation oran apparatus having the equivalent function) is used to performmeasurement by a method of adding about 0.1 g of a sample, a surfactant,and 1 to 2 mL of distilled water to a beaker, sufficiently stirringthem, injecting the mixture into a water stirring tank, measuring aluminous intensity distribution 64 times at an interval of 2 sec, andanalyzing particle size distribution data.

3) Electrolyte-Containing Layer

The electrolyte-containing layer may be arranged between the positiveand negative electrodes. The electrolyte-containing layer electricallyinsulates the positive and negative electrodes from each other, and hasionic conductivity. Examples of the electrolyte-containing layer includea composite electrolyte layer and a separator holding a liquid or gelelectrolyte. Both the composite electrolyte layer and the separator maybe used for the electrolyte-containing layer.

If the liquid electrolyte and/or gel electrolyte is used, the separatormay be arranged between the positive and negative electrodes. As theseparator, for example, a synthetic resin nonwoven fabric, polyethyleneporous film, polypropylene porous film, cellulose nonwoven fabric, orthe like can be used.

4) Container Member

As the container member, a laminated film container or a metal containercan be used. The container can have a shape conforming to the form ofthe secondary battery.

The thickness of the laminated film preferably falls within the range of0.5 mm or less. A more preferable range is 0.2 mm or less. The lowerlimit value of the thickness of the laminated film is desirably set to0.01 mm.

On the other hand, the plate thickness of the metal container preferablyfalls within the range of 0.5 mm or less. A more preferable range is 0.3mm or less. The lower limit value of the plate thickness of the metalcontainer is desirably set to 0.05 mm.

As the laminated film, for example, a multilayer film including a metallayer and a resin layer covering the metal layer can be used. To reducethe weight, the metal layer is preferably an aluminum foil or analuminum alloy foil. The purity of the aluminum foil is preferably 99.5wt % or more. The resin layer reinforces the metal layer, and can beformed from a polymer such as polypropylene (PP), polyethylene (PE),nylon, or polyethylene terephthalate (PET).

The laminated film container can be obtained by sealing laminated filmsusing heat seal.

As the metal container, a metal can made of aluminum, an aluminum alloy,iron, or stainless steel and having a rectangular or cylindrical shapeis usable. The metal container is desirably made of aluminum or analuminum alloy. As the aluminum alloy, an alloy containing at least oneelement selected from the group consisting of manganese, magnesium,zinc, and silicon. The aluminum purity of the alloy is preferably 99.8wt % or less. When the strength of the metal can made of an aluminumalloy significantly increases, it is possible to thin the thickness ofthe can. As a result, it is possible to implement a thin and lightweightbattery having a high output and excellent heat dissipation.

The metal container can be sealed by a laser. Therefore, it is possibleto reduce the volume of a sealing portion, as compared to the laminatedfilm container, thereby improving the energy density.

The secondary battery according to the embodiment can be applied tosecondary batteries in various forms such as a rectangular type, acylindrical type, a flat type, a thin type, and a coin type. Thesecondary battery preferably has a bipolar structure. This isadvantageous in producing, by one battery, a secondary battery having avoltage equal to that of a plurality of serial batteries. A bipolarbattery including a composite electrolyte can prevent a short circuitcaused by an electrolyte.

An example of the secondary battery according to the embodiment will bedescribed with reference to FIGS. 1, 2, 3, 4, and 5.

FIGS. 1 and 2 show an example of the secondary battery using a metalcontainer.

An electrode group 1 is stored in a rectangular tubular metal container2. The electrode group 1 has a structure formed by spirally winding thepositive electrode active material-containing layer of a positiveelectrode 3 and the negative electrode active material-containing layerof a negative electrode 4 with an electrolyte-containing layer 5interposing therebetween so as to form a flat shape. Theelectrolyte-containing layer 5 covers the surface of the positiveelectrode active material-containing layer or the negative electrodeactive material-containing layer. As shown in FIG. 2, a strip-shapedpositive electrode lead 6 is electrically connected to each of aplurality of portions at an end of the positive electrode 3 located onan end face of the electrode group 1. A strip-shaped negative electrodelead 7 is electrically connected to each of a plurality of portions atan end of the negative electrode 4 located on the end face. Theplurality of positive electrode leads 6 are bundled, and in this state,electrically connected to a positive electrode tab 8. A positiveelectrode terminal is formed from the positive electrode leads 6 and thepositive electrode tab 8. In addition, the negative electrode leads 7are bundled, and in this state, connected to a negative electrode tab 9.A negative electrode terminal is formed from the negative electrodeleads 7 and the negative electrode tab 9. A sealing plate 10 made of ametal is fixed to the opening portion of the metal container 2 bywelding or the like. The positive electrode tab 8 and the negativeelectrode tab 9 are extracted to the outside from outlet holes formed inthe sealing plate 10, respectively. The inner surface of each outlethole of the sealing plate 10 is covered with an insulating member 11 toavoid a short circuit caused by contact between the positive electrodetab 8 and the negative electrode tab 9.

FIGS. 3 and 4 show an example of a secondary battery using a containermember made of a laminated film.

As shown in FIGS. 3 and 4, the flat wound electrode group 1 is stored ina sack-shaped container member 12 made of a laminated film including ametal layer interposing between two resin films. The flat woundelectrode group 1 is formed by spirally winding a stacked structureobtained by stacking the negative electrode 4, an electrolyte-containinglayer 15, the positive electrode 3, and the electrolyte-containing layer15 from the outside, and pressing the stacked structure. The outermostnegative electrode 4 has an arrangement in which a negative electrodelayer (negative electrode active material-containing layer) 4 bcontaining a negative electrode active material on one surface on theinner side of a negative electrode current collector 4 a, as shown inFIG. 4, and the remaining negative electrodes 4 are arranged by formingthe negative electrode layers 4 b on both surfaces of the negativeelectrode current collector 4 a. The positive electrode 3 is arranged byforming positive electrode layers (positive electrode activematerial-containing layers) 3 b on both surfaces of a positive electrodecurrent collector 3 a.

Near the outer end of the wound electrode group 1, a negative electrodeterminal 13 is connected to the negative electrode current collector 4 aof the outermost negative electrode 4, and a positive electrode terminal14 is connected to the positive electrode current collector 3 a of thepositive electrode 3 on the inner side. The negative electrode terminal13 and the positive electrode terminal 14 extend outward from theopening portion of the sack-shaped container 12. The opening portion ofthe sack-shaped container 12 is heat-sealed, thereby sealing the woundelectrode group 1. At the time of heat-sealing, the negative electrodeterminal 13 and the positive electrode terminal 14 are sandwiched by thesack-shaped container member 12 in the opening portion.

Examples of each of the electrolyte-containing layers 5 and 15 include acomposite electrolyte layer and a separator holding a liquid or gelelectrolyte. Both the composite electrolyte layer and the separator maybe used for the electrolyte-containing layer 5 or 15.

A secondary battery having a bipolar structure will be described next.The secondary battery further includes a current collector with thefirst surface and the second surface located on the opposite side of thefirst surface. As the current collector, a current collector similar tothe positive or negative electrode current collector of the secondarybattery is usable. The secondary battery has a bipolar structure inwhich a positive electrode active material-containing layer is formed onthe first surface of the current collector, and a negative electrodeactive material-containing layer is formed on the second surface. Acomposite electrolyte layer exists on the surface of at least one of thepositive electrode active material-containing layer and the negativeelectrode active material-containing layer. As a result, at least partof the composite electrolyte layer is located between the positiveelectrode active material-containing layer and the negative electrodeactive material-containing layer. As the positive and negative electrodeactive material-containing layers, positive and negative electrodeactive material-containing layers similar to those described for thesecondary battery are usable.

The composite electrolyte improves the ionic conductivity in thesecondary battery having the bipolar structure. This can implement ahigh-voltage secondary battery by one single battery without connectinga plurality of single batteries in series. Furthermore, the compositeelectrolyte can take the form of a gel, and thus prevent a short circuitcaused via an electrolytic solution in the bipolar cell.

FIG. 5 shows an example of a bipolar secondary battery. The secondarybattery shown in FIG. 5 includes a metal container 31, an electrode body32 having a bipolar structure, a sealing plate 33, a positive electrodeterminal 34, and a negative electrode terminal 35. The metal container31 has a bottomed square tubular shape. As the metal container, a metalcontainer similar to that described for the secondary battery is usable.The electrode body 32 having the bipolar structure includes a currentcollector 36, a positive electrode layer (positive electrode activematerial-containing layer) 37 stacked on one surface (first surface) ofthe current collector 36, and a negative electrode layer (negativeelectrode active material-containing layer) 38 stacked on the othersurface (second surface) of the current collector 36. A compositeelectrolyte layer 39 is arranged between the electrode bodies 32 eachhaving the bipolar structure. The positive electrode terminal 34 and thenegative electrode terminal 35 are fixed to the sealing plate 33 via aninsulating member 42. One end of a positive electrode lead 40 iselectrically connected to the positive electrode terminal 34 and theother end is electrically connected to the current collector 36. One endof a negative electrode lead 41 is electrically connected to thenegative electrode terminal 35 and the other end is electricallyconnected to the current collector 36.

Since the secondary battery according to the above-described secondembodiment includes the positive electrode according to the firstembodiment, it is possible to obtain excellent charge-and-dischargecycle life, discharge rate performance, and low-temperature performance.

Third Embodiment

A battery module according to the third embodiment includes a pluralityof secondary batteries according to the second embodiment.

Examples of the battery module can include a battery module includingplural single batteries electrically connected in series and/or inparallel as constituent units, and a battery module including a firstunit formed from plural single batteries electrically connected inseries or a second unit formed from plural single batteries electricallyconnected in parallel. The battery module may include at least one ofthe forms.

Examples of the form in which plural secondary batteries areelectrically connected in series and/or in parallel include a form inwhich plural batteries each including a container member areelectrically connected in series and/or in parallel and a form in whichplural electrode groups or bipolar electrode bodies stored in a commoncase are electrically connected in series and/or in parallel. As adetailed example of the former, the positive electrode terminals and thenegative electrode terminals of plural secondary batteries are connectedby bus bars made of a metal (for example, aluminum, nickel, or copper).As a detailed example of the latter, plural electrode groups or bipolarelectrode bodies are stored in one case in a state in which they areelectrochemically insulated by partitions, and the electrode groups orbipolar electrode bodies are electrically connected in series. Ifsecondary batteries are used, when the number of batteries electricallyconnected in series falls within the range of 5 to 7, satisfactoryvoltage compatibility with a lead storage battery can be obtained. Tofurther improve the voltage compatibility with a lead storage battery, astructure including five or six single batteries connected in series ispreferable.

As the case storing the battery module, a metal can made of an aluminumalloy, iron, stainless steel, or the like, a plastic container, or thelike can be used. The plate thickness of the container is desirably 0.5mm or more.

An example of a battery module will be described with reference to FIG.6. A battery module 21 shown in FIG. 6 includes, as single batteries, aplurality of rectangular secondary batteries 22 ₁ to 22 ₅ shown inFIG. 1. A positive electrode tab 8 of the battery 22 ₁ and a negativeelectrode tab 9 of the battery 22 ₂ located adjacent to the battery 22 ₁are electrically connected by a lead or bus bar 23. In addition, apositive electrode tab 8 of the battery 22 ₂ and a negative electrodetab 9 of the battery 22 ₃ located adjacent to the battery 22 ₂ areelectrically connected by a lead or bus bar 23. The batteries 22 ₁ to 22₅ are thus electrically connected in series.

Since the battery module according to the above-described thirdembodiment includes the secondary batteries including the positiveelectrode according to the first embodiment, it is possible to obtainexcellent charge-and-discharge cycle life, discharge rate performance,and low-temperature performance.

Fourth Embodiment

A battery pack according to the fourth embodiment can include one orplural secondary batteries (single batteries) according to the secondembodiment. The plural secondary batteries can be electrically connectedin series, in parallel, or in a combination of series connection andparallel connection, thereby forming a battery module. The battery packaccording to the fourth embodiment may include plural battery modules.

The battery pack according to the fourth embodiment can further includea protective circuit. The protective circuit has a function ofcontrolling charge/discharge of a secondary battery. Alternatively, acircuit included in a device (for example, an electronic device or anautomobile) that uses a battery pack as a power supply can be used asthe protective circuit of the battery pack.

The battery pack according to the fourth embodiment can further includeas an external power distribution terminal an external energizingterminal. The external power distribution terminal is configured tooutput a current from the secondary battery to the outside and input acurrent to the secondary battery. In other words, when the battery packis used as a power supply, a current is supplied to the outside via theexternal power distribution terminal. Additionally, when charging thebattery pack, a charge current (including regenerative energy of thepower of a vehicle such as an automobile) is supplied to the batterypack via the external power distribution terminal.

FIGS. 7 and 8 show an example of a battery pack 50. FIG. 7 is anexploded perspective view of an example of the battery pack 50. FIG. 8is a block diagram showing an electric circuit of the battery pack 50.

The battery pack 50 shown in FIGS. 7 and 8 includes single batteries 51.The single batteries 51 are flat nonaqueous electrolyte batteriesdescribed with reference to FIG. 3.

Plural single batteries 51 are stacked so that the negative electrodeterminals 13 and the positive electrode terminals 14 extended outsideare arranged in the same direction, and fastened with an adhesive tape52 to configure a battery module 53. The single batteries 51 areelectrically connected to each other in series as shown in FIG. 8.

A printed wiring board 54 is arranged to face opposite to the side planeof the single battery 51 where the negative electrode terminal 13 andthe positive electrode terminal 14 extend out from. A thermistor 55, aprotective circuit 56, and a power distribution terminal 57 to anexternal device as the external power distribution terminal are mountedon the printed wiring board 54 as shown in FIG. 8. An electricinsulating plate (not shown) is attached to the surface of the printedwiring board 54 facing the battery module 53 to avoid unnecessaryconnection of the wires of the battery module 53.

A positive electrode-side lead 58 is connected to the positive electrodeterminal 14 located at the bottom layer of the battery module 53 and thedistal end of the lead 58 is inserted into a positive electrode-sideconnector 59 of the printed wiring board 54 so as to be electricallyconnected. A negative electrode-side lead 60 is connected to thenegative electrode terminal 13 located at the top layer of the batterymodule 53 and the distal end of the lead 60 is inserted into an negativeelectrode-side connector 61 of the printed wiring board 54 so as to beelectrically connected. The connectors 59 and 61 are connected to theprotective circuit 56 through wires 62 and 63 formed on the printedwiring board 54.

The thermistor 55 detects the temperature of the single batteries 51,and the detection signal is sent to the protective circuit 56. Theprotective circuit 56 can shut down a plus-side wire 64 a and aminus-side wire 64 b between the protective circuit 56 and the powerdistribution terminal 57 to an external device under a predeterminedcondition. The predetermined condition indicates, for example, the casewhere the temperature detected by the thermistor 55 becomes apredetermined temperature or more. Another example of the predeterminedcondition indicates the case where the over-charge, over-discharge, orover-current of the single batteries 51 is detected. The detection ofthe over-charge and the like is performed on each of the singlebatteries 51 or the entire battery module 53. When each of the singlebatteries 51 is detected, the battery voltage may be detected, or apositive electrode or negative electrode potential may be detected. Inthe latter case, a lithium electrode to be used as a reference electrodeis inserted into each of the single batteries 51. In the case of thebattery pack of FIGS. 7 and 8, wires 65 for voltage detection areconnected to each of the single batteries 51. Detection signals are sentto the protective circuit 56 through the wires 65.

Protective sheets 66 made of rubber or resin are arranged on three sideplanes of the battery module 53 except the side plane from which thepositive electrode terminal 14 and the negative electrode terminal 13protrude out from.

The battery module 53 is housed in a housing container 67 together witheach of the protective sheets 66 and the printed wiring board 54. Thatis, the protective sheets 66 are arranged on both internal surfaces in along side direction and on one internal surface in a short sidedirection of the housing container 67. The printed wiring board 54 isarranged on the internal surface on the opposite side in a short sidedirection. The battery module 53 is located in a space surrounded by theprotective sheets 66 and the printed wiring board 54. A lid 68 isattached to the upper surface of the housing container 67.

In order to fix the battery module 53, a heat-shrinkable tape may beused in place of the adhesive tape 52. In this case, the battery moduleis bound by placing the protective sheets on the both sides of thebattery module, revolving the heat-shrinkable tape, and thermallyshrinking the heat-shrinkable tape.

In FIGS. 7 and 8, the form in which the single batteries 51 areconnected in series is shown. However, in order to increase the batterycapacity, the batteries may be connected in parallel. Alternatively, thebatteries may be formed by combining series connection and parallelconnection. The assembled battery pack can be connected in series or inparallel.

The battery pack shown in FIGS. 7 and 8 includes one battery module. Thebattery pack according to the fourth embodiment may include a pluralityof battery modules. The plurality of battery modules are electricallyconnected in series, in parallel, or in a combination of seriesconnection and parallel connection.

The form of the battery pack is appropriately changed in accordance withthe application purpose. The battery pack according to this embodimentis preferably used for an application purpose requiring that excellentcycle performance is obtained at the time of a large currentdischarging. More specifically, the battery pack is used as a powersupply for a digital camera, a battery for a vehicle such as a two- orfour-wheeled hybrid electronic automobile, a two- or four-wheeledelectronic automobile, an electric bicycle, or a railway vehicle (forexample, an electric train), or a stationary battery. Especially, thebattery pack is preferably used as an onboard battery installed in avehicle.

Since the battery pack according to the above-described fourthembodiment includes the secondary battery including the positiveelectrode according to the first embodiment, it is possible to obtainexcellent charge-and-discharge cycle life, discharge rate performance,and low-temperature performance.

Fifth Embodiment

A vehicle according to the fifth embodiment includes one or two or moresecondary batteries according to the second embodiment, the batterymodule according to the third embodiment, or the battery pack accordingto the fourth embodiment.

According to the fifth embodiment, in a vehicle such as an automobile inwhich the battery pack is installed, the battery pack is configured to,for example, collect regenerative energy of the power of the vehicle.The vehicle may include a mechanism of converting the kinetic energy ofthe vehicle into regenerative energy.

FIG. 9 shows an example of an automobile including a battery pack as anexample according to the fourth embodiment.

An automobile 71 shown in FIG. 9 includes a battery pack 72 as anexample according to the fourth embodiment, that is installed in anengine room on the front side of the vehicle body. The mount position ofthe battery pack in the automobile is not limited to the engine room.For example, the battery pack can be installed in the rear side of thevehicle body or under a seat of the automobile. FIG. 10 is a viewschematically showing the arrangement of an example of the vehicleaccording to the embodiment. A vehicle 300 shown in FIG. 10 is anelectric automobile.

The vehicle 300, shown in FIG. 10, includes a vehicle body, a vehiclepower source 301, a vehicle ECU (electric control unit) 380, which is amaster controller of the vehicle power source 301, an external terminal(an external power connection terminal) 370, an inverter 340, and adrive motor 345.

The vehicle 300 includes the vehicle power source 301, for example, inthe engine compartment, in the rear sections of the automobile body, orunder a seat. In FIG. 10, the position of the vehicle power source 301installed in the vehicle 300 is schematically shown.

The vehicle power source 301 includes plural (for example, three)battery packs 312 a, 312 b and 312 c, BMU (a battery management unit)311, and a communication bus 310.

The three battery packs 312 a, 312 b and 312 c are electricallyconnected to each other in series. The battery pack 312 a includes abattery module 314 a and a battery module monitoring unit (VTM: voltagetemperature monitoring) 313 a. The battery pack 312 b includes a batterymodule 314 b, and a battery module monitoring unit 313 b. The batterypack 312 c includes a battery module 314 c, and a battery modulemonitoring unit 313 c. The battery packs 312 a, 312 b and 312 c can eachbe independently removed, and may be exchanged by a different batterypack.

Each of the battery modules 314 a to 314 c includes plural singlebatteries connected to each other in series.

At least one of the plural single batteries is the secondary batteryaccording to the embodiment. The battery modules 314 a to 314 c eachperform charging and discharging through a positive electrode terminal316 and a negative electrode terminal 317.

In order to collect information concerning security of the vehicle powersource 301, the battery management unit 311 performs communication withthe battery module monitoring units 313 a to 313 c and collectsinformation such as voltages or temperatures of the single batteriesincluded in the battery modules 314 a to 314 c included in the vehiclepower source 301.

The communication bus 310 is connected between the battery managementunit 311 and the battery module monitoring units 313 a to 313 c. Thecommunication bus 310 is configured so that multiple nodes (i.e., thebattery management unit and one or more battery module monitoring units)share a set of communication lines. The communication bus 310 is, forexample, a communication bus configured based on CAN (Control AreaNetwork) standard.

The battery module monitoring units 313 a to 313 c measure a voltage anda temperature of each single battery in the battery modules 314 a to 314c based on communications from the battery management unit 311. It ispossible, however, to measure the temperatures only at several pointsper battery module, and the temperatures of all of the single batteriesneed not be measured.

The vehicle power source 301 may also have an electromagnetic contactor(for example, a switch unit 333 shown in FIG. 10) for switchingconnection between the positive electrode terminal and the negativeelectrode terminal. The switch unit 333 includes a precharge switch (notshown), which is turned on when the battery modules 314 a to 314 c arecharged, and a main switch (not shown), which is turned on when batteryoutput is supplied to a load. The precharge switch and the main switchinclude a relay circuit (not shown), which is turned on or off based ona signal supplied to a coil located near the switch element.

The inverter 340 converts an inputted DC (direct current) voltage to athree-phase AC (alternate current) high voltage for driving a motor.Three-phase output terminal(s) of the inverter 340 is connected to eachthree-phase input terminal of the drive motor 345. The inverter 340controls an output voltage based on control signals from the batterymanagement unit 311 or the vehicle ECU 380, which controls the wholeoperation of the vehicle.

The drive motor 345 is rotated by electric power supplied from theinverter 340. The rotation is transferred to an axle and driving wheelsW, for example, through a differential gear unit.

The vehicle 300 also includes a regenerative brake mechanism, though notshown. The regenerative brake mechanism rotates the drive motor 345 whenthe vehicle 300 is braked, and converts kinetic energy into regenerativeenergy, as electric energy. The regenerative energy, recovered in theregenerative brake mechanism, is inputted into the inverter 340 andconverted to direct current. The direct current is inputted into thevehicle power source 301.

One terminal of a connecting line L1 is connected through a currentdetector (not shown) in the battery management unit 311 to the negativeelectrode terminal 317 of the vehicle power source 301. The otherterminal of the connecting line L1 is connected to a negative electrodeinput terminal of the inverter 340.

One terminal of a connecting line L2 is connected through the switchunit 333 to the positive electrode terminal 316 of the vehicle powersource 301. The other terminal of the connecting line L2 is connected toa positive electrode input terminal of the inverter 340.

The external terminal 370 is connected to the battery management unit311. The external terminal 370 is able to connect, for example, to anexternal power source.

The vehicle ECU 380 cooperatively controls the battery management unit311 together with other units in response to inputs operated by a driveror the like, thereby performing the management of the whole vehicle.Data concerning the security of the vehicle power source 301, such as aremaining capacity of the vehicle power source 301, are transferredbetween the battery management unit 311 and the vehicle ECU 380 throughcommunication lines.

The vehicle according to the fifth embodiment includes the battery packwith the secondary batteries according to the embodiment, and thebattery pack (for example, the battery packs 312 a, 312 b, and 312 c)has excellent charge-and-discharge cycle life, discharge rateperformance, and low-temperature performance. Therefore, a reliablevehicle having excellent charge-and-discharge performance can beobtained. In addition, each battery pack is inexpensive and safe. It istherefore possible to suppress the cost of the vehicle and increase thesafety.

EXAMPLES

Examples of the present embodiment will be described in detail belowwith reference to the accompanying drawings. However, the presentembodiment is not limited to the examples to be described below.

Example 1

Secondary particles of LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ having an averagesecondary particle size of 5 μm were prepared as positive electrodeactive material particles. 2 wt % of particles ofLi_(1.3)Al_(0.3)Zr_(1.7) (PO₄)₃ having an average primary particlediameter of 0.4 μm serving as inorganic solid powder, 0.5 wt % ofcellulose nanofiber having an average fiber diameter of 10 nm, 5 wt % ofgraphite powder serving as a conductive agent, and 3 wt % of PVdFserving as a binder were mixed with 89.5 wt % of positive electrodeactive material particles. The obtained mixture was dispersed in ann-methylpyrolidone (NMP) solvent to prepare a slurry. After that, theobtained slurry was applied to both surfaces of a 15 μm-thick aluminumalloy foil (purity of 99%), dried, and pressed, thereby producing apositive electrode having an electrode density of 3.1 g/cm³.

As negative electrode active material particles, particles of Li₄Ti₅O₁₂having an average primary particle size of 0.6 μm and a specific surfacearea of 10 m²/g were prepared. The negative electrode active materialparticles, the graphite powder having an average particle size of 6 μmand serving as a conductive agent, and PVdF serving as a binder weremixed at a weight ratio of 95:3:2 and dispersed in an n-methylpyrolidone(NMP) solvent. The mixture was stirred using a ball mill at a rotationalspeed of 1,000 rpm for a stirring time of 2 hours to prepare a slurry.The obtained slurry was applied to a 15 μm-thick aluminum alloy foil(purity of 99.3%), dried, and heat-pressed, thereby producing a negativeelectrode having a negative electrode layer density of 2.2 g/cm³. Theporosity of the negative electrode except for the current collector was35%.

A composite electrolyte layer was produced as an electrolyte-containinglayer. An aqueous solution obtained by dispersing 5 wt % of particles ofLi_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ having an average particle size(diameter) of 1 μm and 0.5 wt % of cellulose nanofiber having an averagefiber diameter of 10 nm was applied to both principal surfaces of thepositive electrode active material-containing layer and both principalsurfaces of the negative electrode active material-containing layer, anddried, thereby obtaining a electrolyte-containing layer precursor. Anelectrode group was obtained by stacking the positive electrode and thenegative electrode so that the electrolyte-containing layer precursorinterposed between the positive electrode active material-containinglayer and the negative electrode active material-containing layer, andspirally winding the stacked structure in a flat shape. The obtainedelectrode group was stored in a thin metal can having a thickness of0.25 mm and made of an aluminum alloy (Al purity of 99%).

LiPF₆ was dissolved, at a concentration of 1 M, in a solvent obtained bymixing propylene carbonate (PC) and diethyl carbonate (DEC) at a volumeratio of PC:DEC=1:2, thereby obtaining a liquid nonaqueous electrolyte.80 wt % of the liquid nonaqueous electrolyte and 20 wt % of a polymercontaining polyacrylonitrile (PAN) were mixed. The obtained mixedsolution was injected into the metal can to permeate the pores of thepositive and negative electrodes. After that, a heat treatment wasperformed at 60° C. for 24 hours, thereby forming a 5 μm-thick gelcomposite electrolyte layer. In the composite electrolyte layer, theweight ratio of particles of Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃, a gelpolymer containing a nonaqueous electrolytic solution and a polymersolution of polyacrylonitrile (PAN), and cellulose nanofiber was 94:4:2.

By performing the above steps, a rectangular nonaqueous electrolytesecondary battery having the above-described structure shown in FIG. 1and having a thickness of 13 mm, a width of 62 mm, and a height of 96 mmwas obtained.

Examples 2 to 22 and Comparative Examples 1 to 8

A secondary battery was produced in accordance with the same procedureas in Example 1 except that a positive electrode active material, anegative electrode active material, the composition and content ofinorganic solid particles, and the average fiber diameter and content ofpolymer fibers are as shown in Tables 1 and 2 below, the composition ofa positive electrode active material-containing layer is as shown inTables 3 and 4, the average secondary particle size of the positiveelectrode active material according to Example 12 is set to 10 μm, theaverage secondary particle size of the positive electrode activematerial according to Example 13 is set to 5 μm, and the averagesecondary particle size and average primary particle size of thenegative electrode active material according to each of Examples 14 to19 are respectively set to 8 μm and 0.6 μm.

Example 23

An electrode group was produced in accordance with the same procedure asin Example 1, and stored in a thin metal can similar to that describedin Example 1.

A liquid nonaqueous electrolyte having the same composition as inExample 1 was injected into the metal can to permeate the pores ofpositive and negative electrodes. After that, the metal can was sealed,thereby obtaining a rectangular nonaqueous electrolyte secondary batteryhaving the structure shown in FIG. 1 and the same size as in Example 1.

Example 24

Instead of the electrolyte-containing layer precursor, a separator madeof a cellulose fiber nonwoven fabric having a thickness of 20 μm wasused. An electrode group was produced by stacking a positive electrodeand a negative electrode so that the separator interposed between apositive electrode active material-containing layer and a negativeelectrode active material-containing layer, and spirally winding thestacked structure in a flat shape, and stored in a thin metal cansimilar to that described in Example 1.

A liquid nonaqueous electrolyte having the same composition as inExample 1 was injected into the metal can to permeate the pores of thepositive and negative electrodes. After that, the metal can was sealed,thereby obtaining a rectangular nonaqueous electrolyte secondary batteryhaving the structure shown in FIG. 1 and the same size as in Example 1.

Each of the obtained secondary batteries according to the examples andcomparative examples was charged up to 2.9 V at 25° C. with a constantcurrent of 12 A, and then charged with a constant voltage of 2.9 V. Whenthe current value reached 0.05 A, charging ended. Subsequently, thedischarge capacity at the time of discharging to 1.5 V with 6 A wasmeasured and set as a discharge capacity at 25° C. The results are shownin Tables 5 and 6.

As the charge-and-discharge cycle performance, a secondary battery wascharged up to 2.9 V with a constant current of 12 A at 25° C., and thencharged with a constant voltage of 2.9 V. When the current value reached0.05 A, charging ended. Subsequently, the secondary battery wasdischarged to 1.5 V with 6 A. A capacity retention ratio was measuredafter repeating this charge-and-discharge cycle 5,000 times. Defining adischarge capacity in the first cycle as 100%, a value representing adischarge capacity in the 5,000th cycle is shown, in Tables 5 and 6, asa capacity retention ratio in the 5,000th cycle.

As the large-current discharge performance, a second battery was chargedup to 2.9 V at 25° C. with a constant current of 12 A, and then chargedwith a constant voltage of 2.9 V. When the current value reached 0.05 A,charging ended. A capacity retention ratio at the time of 5 C (30 A)discharge was measured. The capacity retention ratio was calculated bydefining a capacity at the time of 0.2 C (1.2 A) discharge as 100%.

As the low-temperature discharge performance, the secondary battery wascharged up to 2.9 V with a constant current of 12 A at 25° C., and thencharged with a constant voltage of 2.9 V. When the current value reached0.05 A, charging ended. Subsequently, the capacity retention ratio atthe time of discharging with 1.2 A at −30° C. was measured. The capacityretention ratio was calculated by defining a discharge capacity at 25°C. as 100%.

These measurement results are shown in Tables 5 and 6 below.

TABLE 1 Polymer fiber Negative Inorganic solid particles in Averageelectrode positive electrode fiber Positive electrode active Contentdiameter Content active material material Composition (wt %) (nm) (wt %)Example 1 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 0.5 Example 2LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 250 0.5 Example 3 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 100 0.5 Example 4LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 25 0.5 Example 5 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 1 0.5 Example 6LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 110 0.5 Example 7 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 5 10 0.5 Example 8LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.2)Ca_(0.1)Zr_(1.9)(PO₄)₃ 1010 1 Example 9 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 1.5 Example 10LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 210 2.5 Example 11 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 1 Example 12 Olivine type Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 1 LiMn_(0.8)Fe_(0.2)PO₄ Example 13LiMn₂O₄ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 1 Example 14LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ TiNb₂O₇ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 101 Example 15 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ TiNb₂O₇ Li₃PO₄ 3 10 1 Example16 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ TiNb₂O₇ Al₂O₃ 3 10 1 Example 17LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ TiNb₂O₇ ZrO₂ 3 10 1 Example 18LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ TiNb₂O₇ TiO₂ 3 10 1 Example 19LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ TiNb₂O₇ AlPO₄ 3 10 1

TABLE 2 Polymer fiber Negative Inorganic solid particles in Averageelectrode positive electrode fiber Positive electrode active Contentdiameter Content active material material Composition (wt %) (nm) (wt %)Example 20 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 0.2 Example 21LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 210 5 Example 22 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 0.5 10 0.5 Example 23LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 210 0.5 Example 24 LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 2 10 0.5 ComparativeLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ — — — 0 Example 1 ComparativeLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃1.5 — 0 Example 2 Comparative LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Al₂O₃ 1 — 0 Example 3 Comparative LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂Li₃PO₄ 1 — 0 Example 4 Comparative LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂Li₄Ti₅O₁₂ Li_(1.3)Al_(0.3)Zr_(1.7)(PO₄)₃ 1 — 0 Example 5 ComparativeLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ — — 10 5 Example 6 ComparativeLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ — — 10 0.05 Example 7 ComparativeLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Li₄Ti₅O₁₂ — — 500 1 Example 8

TABLE 3 Composition of positive electrode active material-containinglayer Positive electrode active Inorganic material solid PolymerConductive particles particles fiber agent Binder (wt %) (wt %) (wt %)(wt %) (wt %) Example 1 89.5 2 0.5 5 3 Example 2 89.5 2 0.5 5 3 Example3 89.5 2 0.5 5 3 Example 4 89.5 2 0.5 5 3 Example 5 89.5 2 0.5 5 3Example 6 89.5 1 0.5 6 3 Example 7 88.5 5 0.5 4 2 Example 8 83.0 10 1 42 Example 9 89.5 2 1.5 5 2 Example 10 89.5 2 2.5 4 2 Example 11 89.5 2 15 2.5 Example 12 89.5 2 1 5 2.5 Example 13 89.5 2 1 5 2.5 Example 1489.5 2 1 5 2.5 Example 15 89.5 3 1 4 2.5 Example 16 89.5 3 1 4 2.5Example 17 89.5 3 1 4 2.5 Example 18 89.5 3 1 4 2.5 Example 19 89.5 3 14 2.5

TABLE 4 Composition of positive electrode active material-containinglayer Positive electrode active Inorganic material solid PolymerConductive particles particles fiber agent Binder (wt %) (wt %) (wt %)(wt %) (wt %) Example 20 89.5 2 0.2 5 3.3 Example 21 89.5 2 5 2.5 1.0Example 22 89.5 0.5 0.5 6.5 3 Example 23 89.5 2 0.5 5 3 Example 24 89.52 0.5 5 3 Comparative 89.5 — 0 5 5.5 Example 1 Comparative 80 15 0 2 3Example 2 Comparative 89.5 1 0 4.0 5.5 Example 3 Comparative 89.5 1 04.0 5.5 Example 4 Comparative 89.5 1 0 4.0 5.5 Example 5 Comparative89.5 — 5 4.5 1.0 Example 6 Comparative 89.5 — 0.05 7.4 3.05 Example 7Comparative 89.5 — 1 4.0 5.5 Example 8

TABLE 5 Capacity Capacity Discharge Capacity retention retentioncapacity retention ratio (%) ratio (%) (Ah) at ratio (%) in of 10 C ofdischarge 25° C. 5,000th cycle discharge at −30° C. Example 1 6.0 95 9075 Example 2 6.0 94 80 70 Example 3 6.0 90 75 65 Example 4 6.0 95 90 75Example 5 6.0 94 85 75 Example 6 5.9 95 92 78 Example 7 5.8 95 93 78Example 8 5.9 95 90 78 Example 9 5.8 95 90 78 Example 10 5.7 92 85 75Example 11 6.0 93 88 75 Example 12 6.0 93 85 75 Example 13 5.0 93 94 80Example 14 7.0 90 85 75 Example 15 7.0 88 80 73 Example 16 6.8 87 78 70Example 17 6.8 86 76 70 Example 18 6.8 86 77 70 Example 19 6.9 87 78 73

TABLE 6 Capacity Capacity Discharge Capacity retention retentioncapacity retention ratio (%) ratio (%) (Ah) at ratio (%) in of 10 C ofdischarge 25° C. 5,000th cycle discharge at −30° C. Example 20 6.0 93 9072 Example 21 6.0 96 90 78 Example 22 6.0 90 85 73 Example 23 6.0 95 9075 Example 24 6.0 92 85 65 Comparative 5.9 88 60 60 Example 1Comparative 5.5 80 52 52 Example 2 Comparative 5.8 88 60 60 Example 3Comparative 5.8 89 60 60 Example 4 Comparative 5.8 85 62 62 Example 5Comparative 5.5 85 50 58 Example 6 Comparative 5.9 88 60 60 Example 7Comparative 5.8 86 60 58 Example 8

As is apparent from Tables 5 and 6, Examples 1 to 15 of the secondarybatteries are more excellent in balance among the cycle lifeperformance, discharge rate performance, and low-temperature dischargeperformance, as compared to Comparative Examples 1 to 8. As can be seenfrom comparison between Examples 1 and 23, Example 23 including theliquid electrolyte instead of the gel electrolyte can obtain cycle lifeperformance, discharge rate performance, and low-temperature dischargeperformance that are equivalent to those in Example 1. Furthermore, ascan be seen from comparison between Examples 1 and 24, Example 1including the composite electrolyte layer is more excellent in cyclelife performance, discharge rate performance, and low-temperaturedischarge performance, as compared to Example 24 including the separatorinstead of the composite electrolyte layer.

The positive electrode according to at least one of the embodiments orexamples contains polymer fibers having an average fiber diameter of 1nm to 100 nm and inorganic solid particles, thereby making it possibleto decrease the ion conduction resistance in the positive electrode.Therefore, the secondary battery including this positive electrode canimprove the charge-and-discharge cycle life, discharge rate performance,and low-temperature performance.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A positive electrode comprising a positive electrode activematerial-containing layer that contains positive electrode activematerial particles, polymer fibers having an average fiber diameter of 1nm to 100 nm, and inorganic solid particles.
 2. The positive electrodeof claim 1, wherein the polymer fibers include cellulose fibers.
 3. Thepositive electrode of claim 1, wherein the inorganic solid particlesinclude at least one material selected from the group consisting ofAl₂O₃, TiO₂, ZrO₂, and a phosphate compound.
 4. The positive electrodeof claim 3, wherein the phosphate compound has a NASICON structure andis expressed as Li_(1+y)Al_(x)M_(2−x)(PO₄)₃ (M represents at least oneelement selected from the group consisting of Ti, Ge, Sr, Sn, Zr, and Cafor 0≤x≤1 and 0≤y≤1).
 5. The positive electrode of claim 1, wherein acontent of the polymer fibers in the positive electrode activematerial-containing layer ranges from 0.2 wt % to 5 wt %, and a contentof the inorganic solid particles in the positive electrode activematerial-containing layer ranges from 0.5 wt % to 10 wt %.
 6. Thepositive electrode of claim 1, wherein the positive electrode activematerial particles comprise at least one material selected from thegroup consisting of a lithium manganese composite oxide, a lithiumcobalt composite oxide, a lithium nickel composite oxide, a lithiumnickel cobalt composite oxide, a lithium cobalt aluminum compositeoxide, a lithium nickel aluminum composite oxide, a lithium manganesenickel composite oxide having a spinel structure, a lithium manganesecobalt composite oxide, a lithium-containing phosphate compound havingan olivine structure, a fluorinated iron sulfate, and a lithium nickelcobalt manganese composite oxide.
 7. The positive electrode of claim 1,wherein the positive electrode active material particles compriseLi_(x)Ni_(1−y−z)CO_(y)Mn_(z)O₂, where 0<x≤1.1, 0<y≤0.5, 0<z≤0.5,0<1−y−z<1.
 8. A secondary battery comprising: the positive electrodeaccording to claim 1; a negative electrode; and an electrolyte.
 9. Thesecondary battery of claim 8, wherein the negative electrode includes atitanium-containing oxide.
 10. The secondary battery of claim 9, whereinthe titanium-containing oxide includes at least one material selectedfrom the group consisting of a lithium titanium oxide having a spinelstructure, a monoclinic titanium oxide, and a niobium titanium oxide.11. A battery pack comprising the secondary battery according to claim8.
 12. The battery pack of claim 11, further comprising: an externalpower distribution terminal; and a protective circuit.
 13. The batterypack of claim 11, which comprises plural of the secondary battery, andthe secondary batteries are electrically connected in series, inparallel, or in a combination of series connection and parallelconnection.
 14. A vehicle comprising the battery pack according to claim11.
 15. The vehicle of claim 14, further comprising a mechanismconfigured to convert kinetic energy of the vehicle into regenerativeenergy.